The present invention relates to apparatuses and methods for delivering liquid chemical products. It relates more particularly to a method for measuring the amount of liquid present in a container to which means for making said liquid flow from said container to a point of use are in particular connected, in which method the weight Pi of liquid in the container is measured at a time ti, i varying from 0 to n, this measurement being repeated at time ti+1, then at time ti+2, until time tn (where n is an integer greater than 3).
In the fabrication of semiconductors during which a succession of conducting, semiconducting or dielectric layers (possibly having different dielectric constants) is deposited on a substrate or wafer, generally made of silicon, the deposition operations being carried out selectively through layers for masking (partially) certain surfaces, it is becoming increasingly frequent to produce these layers by bringing chemical liquids called “precursors” to the desired points on the surface, said liquids reacting, under the temperature and pressure conditions created in the reactor in which the wafer or wafers are placed, with other products also introduced into the reactor, such as for example gases. Thus, for example, it is common practice at the present time to create SiC or SiOC dielectric layers using a precursor such as tetramethylsilane (4MS), dimethyldimethoxysilane (DMDMOS) or TMCTS. These precursors are chemical products in liquid form under standard temperature and pressure conditions.
Such precursors have a very high added value and are generally supplied in small containers (volume up to 20 liters typically). These source containers conventionally feed one or more containers that ensure delivery to the customer's equipment. For cost reasons, it is critical to optimize the use of the precursor contained in the source container so as to limit any financial loss associated with returning to the supplier a container that still contains the product. The choice of technique for measuring the amount remaining in the source container is therefore of primary importance.
Many techniques are known at the present time for determining the presence of liquid in a container, with greater or lesser accuracy. In general, the following techniques may be mentioned:                detection by absolute measurement of the weight: this method relies exclusively on the reliability of the weight measuring apparatus, which it is well known is not optimal;        detection by a float within a conical inner container in its lower part, possibly coupled to a weight measurement: this method brings the measuring apparatus into contact with the precursor, with the associated contamination and corrosion risks; in addition, it does not allow optimum use of the product;        ultrasonic detection in the delivery line at the outlet of the container, possibly coupled to a weight measurement: this method allows all of the product contained in the container to be used, but it does require a fine adjustment of the instrumentation, which has the drawback of depending on the nature of the product; moreover, the use of this method represents a considerable additional cost; and        detection by a capacitive or optical sensor: these methods require fittings on the drum containing the products, such as an inspection window or a lateral inspection tube.        
However, all these well-known methods have in general one point in common: they do not allow the chemical product to be used practically down to its last drop.
This is because the cost of the liquid chemical products used hitherto has not necessarily justified the presence of sophisticated means for saving a few grams of products and generally the use of a balance has proved to be sufficient with all the inaccuracies in weight associated, for example, with the variable weight of the container itself, even when the container is supposed to be identical, and the very relative accuracy of balances that support weights of several tens of kilograms.